Showing papers by "Javier J. Concepcion published in 2008"

One site is enough. Catalytic water oxidation by [Ru(tpy)(bpm)(OH2)]2+ and [Ru(tpy)(bpz)(OH2)]2+.

Abstract: Water oxidation is a key reaction in natural photosynthesis and in many schemes for artificial photosynthesis. Although metal complexes capable of oxidizing water based on Ru, Mn, and Ir are known, a significant question is whether or not dimeric or higher order structures are required for water oxidation. We report here single-site catalytic water oxidation by the monomeric complexes [Ru(tpy)(bpm)(OH2)]2+ and [Ru(tpy)(bpz)(OH2)]2+ (tpy is 2,2′:6′,2′′-terpyridine; bpm is 2,2′-bipyrimidine; bpz is 2,2′-bipyrazine) by a well-defined mechanism involving RuV═O.

Mechanisms of water oxidation from the blue dimer to photosystem II

Abstract: The blue dimer, cis, cis-[(bpy)2(H2O)Ru(III)ORu(III)(H2O)(bpy)2](4+), is the first designed, well-defined molecule known to function as a catalyst for water oxidation. It meets the stoichiometric requirements for water oxidation, 2H2O --> -4e(-), -4H(+) O-O, by utilizing proton-coupled electron-transfer (PCET) reactions in which both electrons and protons are transferred. This avoids charge buildup, allowing for the accumulation of multiple oxidative equivalents at the Ru-O-Ru core. PCET and pathways involving coupled electron-proton transfer (EPT) are also used to avoid high-energy intermediates. Application of density functional theory calculations to molecular and electronic structure supports the proposal of strong electronic coupling across the micro-oxo bridge. The results of this analysis provide explanations for important details of the descriptive chemistry. Stepwise e(-)/H(+) loss leads to the higher oxidation states [(bpy)2(O)Ru(V)ORu(IV)(O)(bpy)2] (3+) (Ru(V)ORu(IV)) and [(bpy)2(O)Ru(V)ORu(V)(O)(bpy)2](4+) (Ru(V)ORu(V)). Both oxidize water, Ru(V)ORu(IV) stoichiometrically and Ru(V)ORu(V) catalytically. In strongly acidic solutions (HNO3, HClO4, and HSO3CF3) with excess Ce(IV), the catalytic mechanism involves O---O coupling following oxidation to Ru(V)ORu(V), which does not build up as a detectable intermediate. Direct evidence has been found for intervention of a peroxidic intermediate. Oxidation of water by Ru(V)ORu(IV) is far slower. It plays a role late in the catalytic cycle when Ce(IV) is depleted and is one origin of anated intermediates such as [(bpy)2(HO)Ru(IV)ORu(IV)(NO3)(bpy)2](4+), which are deleterious in tying up active components in the catalytic cycle. These intermediates slowly return to [(bpy)2(H2O)Ru(IV)ORu(III)(OH2)(bpy)2](5+) with anion release followed by water oxidation. The results of a recent analysis of water oxidation in the oxygen-evolving complex (OEC) of photosystem II reveal similarities in the mechanism with the blue dimer and significant differences. The OEC resides in the thylakoid membrane in the chloroplasts of green plants, and careful attention is paid in the structure to PCET, EPT, and long-range proton transfer by sequential local proton transfers. The active site for water oxidation is a CaMn 4 cluster, which includes an appended Mn site, Mn(4), where O---O coupling is thought to occur. Photochemical electron transfer results in oxidation of tyrosine Y Z to Y Z (.), which is approximately 7 A from Mn(4). It subsequently oxidizes the OEC through the stepwise stages of the Kok cycle. O---O coupling appears to occur through an initial peroxidic intermediate formed by redox nucleophilic attack of coordinated OH(-) in Ca-OH(-) on Mn (IV)=O.

Mediator-assisted water oxidation by the ruthenium “blue dimer” cis,cis-[(bpy)2(H2O)RuORu(OH2)(bpy)2]4+

Abstract: Light-driven water oxidation occurs in oxygenic photosynthesis in photosystem II and provides redox equivalents directed to photosystem I, in which carbon dioxide is reduced. Water oxidation is also essential in artificial photosynthesis and solar fuel-forming reactions, such as water splitting into hydrogen and oxygen (2 H2O + 4 hν → O2 + 2 H2) or water reduction of CO2 to methanol (2 H2O + CO2 + 6 hν → CH3OH + 3/2 O2), or hydrocarbons, which could provide clean, renewable energy. The “blue ruthenium dimer,” cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+, was the first well characterized molecule to catalyze water oxidation. On the basis of recent insight into the mechanism, we have devised a strategy for enhancing catalytic rates by using kinetically facile electron-transfer mediators. Rate enhancements by factors of up to ≈30 have been obtained, and preliminary electrochemical experiments have demonstrated that mediator-assisted electrocatalytic water oxidation is also attainable.

Probing the localized-to-delocalized transition.

Abstract: Detailed understanding of the transition between localized and delocalized behaviour in mixed valence compounds has been elusive as evidenced by many interpretations of the CreutzTaube ion, [(NH3)5...